Conventionally, in a radio communication system, when a communication terminal apparatus such as a mobile telephone starts radio communication such as packet transmission, the communication terminal apparatus receives a pilot signal periodically transmitted from the base station apparatus, performs open-loop transmission power control (OL-TPC) on an access request signal based on reception quality of the pilot signal, and transmits the access request signal to the base station apparatus using a random access channel (RACH), and when receiving the access request signal, the base station apparatus transmits an access permission signal to the communication terminal apparatus using a forward access channel (FACH). Then, after receiving the access permission signal, the communication terminal apparatus transmits transmission data to the base station apparatus using an uplink data channel.
In such a radio communication system, when a plurality of communication terminal apparatuses transmit access request signals concurrently using the same resource on the RACH, collision of access request signals occurs on the propagation path, and therefore the base station apparatus cannot receive the access request signals, and as a result, does not transmit access permission signals to the communication terminal apparatuses.
Therefore, a technique is developed that, when an access permission signal is not transmitted within a predetermined response waiting period after transmitting the access request signal, the communication terminal apparatus retransmits the access request signal after a lapse of back-off time randomly set using a timing at which the access request signal was previously transmitted as a reference (for example, see Patent Document 1).
FIG. 1 schematically shows the technique as described in Patent Document 1. In FIG. 1, communication terminal apparatuses land 2 first transmit access request signals concurrently on the same sub-channel of the RACH, and therefore, the base station apparatus cannot receive either of the access request signals. Then, an access permission signal is not transmitted from the base station apparatus within a predetermined response waiting period after the first access request signal is transmitted, and therefore both of communication terminal apparatuses 1 and 2 retransmit access request signals to the base station apparatus after a lapse of randomly set back-off time 1 or back-off time 2, respectively.
Thus, in the technique as described in Patent Document 1, when collision of access request signals occurs, by randomly setting the back-off times, the probability of occurrence of collision of retransmitted access request signals is reduced.
Patent Document 1: Japanese Patent Application Laid-Open No. 2000-308148